The discovery of variable expression of a major surface antigen of Giardia has provided an excellent model for the study for genetic change in eukaryotic organisms. We demonstrated antigenic variation by using a cytotoxic monoclonal antibody for a major 170 kD surface antigen to select antigenic variants from cloned populations of WB Giardia trophozoites. By cloning a portion of the gene for the 170 dK surface antigen from a genomic expression library, we obtained evidence that the antigen is from a multigene family which undergoes frequent DNA rearrangements. The DNA sequence of the gene fragment encodes a cysteine-rich (12%) polypeptide and metabolic labeling of Giardia trophozoites verified the high cysteine content of the 170 kD surface antigen (cysteine-rich protein; CRP 170) as well as that of an antigenic variant. CRP 170 is an immunodominant molecule which is produced in large biologic perspectives. We plan to clone the enter CRP 170 gene from a cDNA library and compare the sequence with other known sequences, especially those of cysteine-rich protein from other organisms, to gain insight into the structure and function of the CRP 170 protein. In order to determine how expression of 170 is lost in the antigenic variants, transcription of CRP 170 will be analyzed with determination of the transcription initiation site by primer extension analysis. A genomic clone of this region will be compared to the equivalent region from the plan to map the chromosome(s) containing the CRP 170 gene by using pulsed field electrophoresis and to look for difference between lines expressing and not expressing CRP 170. These approaches are likely to provide information about how expression of a variant surface antigen is turned off. In addition, we plan to clone the cysteine-rich protein gene from an antigenic variant and perform the same types of analyses. This will provide information about the relationship of the cysteine-rich proteins as well as clues to how a new variant antigen gene is turned on. The findings of these studied should yield important information about how Giardia evades the immune response and may have direct relevance for vaccine development.